Prior to erecting certain structures, the nature of the underlying soil should be examined in order to determine the type of footings and foundations required to properly support the structure. Customarily, a Standard Penetration Test or SPT is performed whereby a split-barrel sampler is driven into the soil to obtain a representative soil sample and to measure the resistance of the soil by recording the number of blows required to drive the sampler a specified distance. ASTM Standard D 1586-84 provides, in part, that a hammer weighing 140.+-.2 lb. (63.5.+-.1 kg) be successively lifted and dropped 30.+-.1 in. (0.76 m.+-.25 mm) unimpeded onto an anvil which is attached to the top of a sampling rod string.
The lifting and dropping of the hammer to produce the blow is accomplished by a hammer drop system. ASTM Standard D 1586--84 recognizes and approves several such systems -- rope-cathead, trip, semi-automatic, or automatic hammer drop system, providing that the lifting apparatus must not cause penetration of the sampler while re-engaging and lifting the hammer. The disadvantages inherent in each of the above systems are well known in the industry. For example, the rope-cathead system, while allowing 40-60 blows per minute, may often be inaccurate. The lifting and dropping of the hammer is controlled by an operator who holds onto one end of a rope. The rope is wound once or twice around a revolving drum or cathead and extends therefrom to the weight. By pulling slightly on the rope, the friction between the rope and the revolving drum assists the operator in lifting the hammer. When the hammer has been raised 30 inches, the operator quickly releases the rope, allowing the hammer to drop and strike the anvil. However, an inattentive operator may release the rope too early or too late relative to the 30 inch drop requirement, he may not release the rope quickly enough or completely, and there may be residual friction between the drum and the rope during the drop. Each of these events may change the energy transmitted to the anvil which in turn compromises the accuracy of the test results.
Most of the accuracy of the test is regained by "automatic" hammer drop systems whereby the lifting and dropping of the hammer is performed entirely by some combination of mechanical, electrical and hydraulic components. An example of such systems is described in U.S. Pat. No. 4,405,020 issued to Rassieur wherein a single lifting lug is mounted on a two-sprocket chain drive. The chain, sprockets, hammer and a motor for driving the sprockets are all carried by a hammer guide housing which is held by a carriage which is pivotally and vertically slidably mounted to a vertical member. As the lug comes around the lower sprocket onto the inward side of the drive chain, it engages the lower striking surface of the hammer, lifting the hammer off the anvil and upward through the hammer guide housing. When the lug reaches and moves around the top sprocket, it moves away from the hammer allowing the hammer to free fall and strike the anvil. The drop height of the hammer for each lift/drop cycle is thus determined by the fixed distance between the sprockets.
After impact and the associated incremental drop of the anvil and drill string, the drive chain assembly must be lowered precisely a corresponding amount so that the lug will properly engage the hammer in the next cycle and so that the hammer may be lifted the required 30 inches. A hydraulic system with adjustable relief valve is provided to govern the movement of the carriage and thereby hammer drop system on the vertical member. This system nevertheless requires monitoring by the operator to provide an acceptable level of accuracy of the test. Further, it is not clear that the hydraulic system with adjustable relief valve is completely successful in assuring that the lifting apparatus will not transfer energy to the sampler and cause penetration of the sampler while re-engaging and lifting the hammer.
What is needed is a drive hammer system which is better able to meet the standards and requirements of the Standard Penetration Test.